The present invention is generally directed to a method for producing a semiconductor device and in particular to a method for producing a semiconductor device having a silicon substrate with a diffusion layer containing a diffused impurity or dopant therein.
One conventional technique for producing semiconductor devices utilizes the gaseous diffusion of impurities into selected regions of a semiconductor substrate. This technique requires handling potentially hazardous gasses at high temperatures.
Another common technique for forming a diffused layer having N-type impurities, in a semiconductor device is disclosed by J. Kato and S. Iwamatsu in the Journal of the Electrochemical Society, V. 131, pp. 1145-1152 (1984). In the method disclosed therein, impurity ions are injected into the silicon substrate by means of an ion injection machine. The semiconductor device is then annealed for a short time by using a halogen lamp, thereby forming shallow diffused layers of the impurity.
The conventional methods, including the ion injection method noted above, have several major disadvantages. First, the cost of the gaseous diffusion machines and ion injection machine is quite high. Further, these apparatuses have a complex structure and require considerable maintenance. Further, the rate of diffusion is low, thereby increasing the cost for forming the diffused layers.
Another major disadvantage of the ion injection technique is that when the junction of the diffused layer and the semiconductor substrate is shallow, the resistance of the resulting impurity layer is high. For example, the sheet resistance of a phosphorus diffusion layer having a depth of not more than 0.2 .mu.m is greater than 50 ohms/.quadrature.. If the source and drain of a metal oxide semiconductor field effect transistor (hereinafter a "MOSFET") is formed of a material of this resistance, the switching speed of the transistor is limited thereby, thus preventing devices manufactured using Large Scale Integration (hereinafter "LSI") from operating at high speeds.
Another disadvantage of the ion injection method is that it damages the silicon crystal approximately 500 .ANG. deeper than the diffusion layer. Thus, even if annealing is performed on the device for only a short period of time, accelerated diffusion occurs due to recovery of crystal defects. The resulting junction layer is therefore more than 500 .ANG. deep. Thus, the defects produced in the silicon crystal caused by ion injection limit the possible reduction in size of LSI devices.
Yet another disadvantage of the ion injection method is that the placement of contact holes for connecting metal conductors to the diffused impurity layer is particularly critical. If the contact hole is not placed precisely on the diffused layer, or is on a peripheral portion of the diffused layer, there is an electrical path between the electrical conductor and the substrate which causes leakage currents. Thus, it is necessary to provide large tolerances on the position of the contact holes and the diffused layers to assure proper registry. This also serves to limit density and LSI miniaturization.
Thus, as noted above the conventional method for forming diffused layers is relatively costly, slow and not conducive to high density integration required in LSI and VLSI.
Accordingly, there is a need for a method for producing a semiconductor device having an impurity layer containing a diffused impurity which is low in cost, does not require a long operation time and does not hinder high density integration.